John Trapp on Jeremiah 34:18

Ver. 18. That have tranſgreſſed my Covenant] His Covenant he calleth it by a weighty Emphaſis; becauſe about a buſineſſe by him commanded, and wherein he was engaged, not as a bare ſpectatour, but as a ſevere avenger of their perjury. Hom. Il. l. 3. Tul. de Invent. Liv. l. 1. Virg. Aeneid. 8. Hinc foedus à foedo animali diviſo.

When they cut the Calf in twain] To ſhew the correſpondency of wills whereunto the contracters did bind themſelves: and the puniſhment of diſſection or other violent death, whereunto they ſubmitted themſelves, in caſe they brake promiſe. The riſe of this rite in covenanting See Gen. 15.9, 10, 17. The Heathens uſed the like ceremony, as is to be ſeen in Homer, Cicero, Livy, Virgil; — & caeſa jungebant faedera porcâ. The Romans cut a ſow in twain: and when it was divided, the Faeciales or heralds gave one half to one party, and the other half to the other, and ſaid, So God divide you aſunder if you break this Covenant: and let God do this ſo much the more, as he is more able.